1. Field of the Invention
The invention relates to an electroacoustic calling device comprising an electroacoustic transducer mounted in a housing, said housing having an aperture or a group of apertures in a wall thereof through which aperture(s) sound waves may be emitted from the housing, said device further comprising a closure member having a surface which is movable across the wall of the housing to cover to a greater or lesser extent said aperture(s), the profile of the surface of the closure member and the exterior surface of the wall in the region of the aperture being complementary.
2. Description of the related art
Such devices are used in telephone instruments and may be of the form shown in U.K. Patent Application No. 2082018A or 2115648A. The prior art devices have a closure member which is mounted for pivotal movement with respect to a point on the housing in order to provide a variable closure of one or more apertures which has the effect of controlling the intensity of the sound waves escaping from the aperture(s). However, this arrangement has the disadvantage that any misalignment between the outer surface of the wall of the housing and the contacting surface of the closure member will allow sound to leak out between these surfaces. This means that the minimum sound level will vary from instrument to instrument since the engagement between these surfaces will differ with normal materials and tolerances.